libceph: pass the message pointer instead of loading con->out_msg

libceph: pass the message pointer instead of loading con->out_msg

This pointer is in a register anyway, so let's use that instead of
reloading from memory everywhere.

[ idryomov: formatting ]

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>